Prior U.S. Pat. Nos. 5,264,299 and 5,523,175 assigned to the assignee herein are representative of a prior art approach to sealing, with an elastomeric sealing material, peripheral portions of fuel cell components, particularly components which may be porous. More particularly, column 4 line 1 through column 6 line 49 of the '299 patent describes how a support plate might be assembled with a proton exchange membrane by such a sealing material peripherally joining these components. The '299 patent is hereby incorporated by reference herein.
Peripheral sealing of prior art membrane and plate assemblies generally requires impregnating the outside perimeter of each component with a chemical, such as SARTOMER GRADE 2108 from SARTOMER CO., INC., that requires curing in the absence of oxygen. Such chemicals are hydrophobic in the cured state so as to suffer from continuous contact with the cooling fluid currently favored, namely water as now used in fuel cells generally.
In prior art fuel cells the edges of water transport plates are impregnated with a resin, which when polymerized create an edge seal that prevents leakage of the gaseous reactants, and also serves to prevent infiltration of these reactants into the liquid coolant.
Typically resins that have been used to create such edge seals in water transport plates are SARTOMER 2100 made by SARTOMER CO., INC., and SYLGARD 170 made by the Dow Chemical Corporation. These polymers are somewhat hydrophobic, however, and are not totally wetted by water. Since water is a product of the reaction within the fuel cell, and typically also comprises the coolant in the proton exchange membrane fuel cell (PEM) such characteristics are undesirable for use in such edge seal applications. More particularly, as a consequence of this hydrophobicity is that the interface between the central active area of the water transport plate and the peripheral edge seal portion of the water transport plate may not totally fill with water. Such a situation reduces the sealing effectiveness of the water transport plate, and results in some leakage of reactant gases into the coolant which can result in a undesirable condition.
The edges of the porous catalyst support plates are impregnated to create edge seals also. One commonly used compound for such impregnation is a silicone rubber. However, the silicone rubber although resulting in an effective seal within the catalyst support plate does not exhibit good adhesion with the proton exchange membrane (PEM). This poor adhesion tends to permit reactant leakage at the interface between these components and allows the PEM to shrink away from the edge of the catalyst support plate, particularly if the water content of the proton exchange membrane is reduced due to the conditions of the PEM operating fuel cell. The shrinkage of the proton exchange membrane can result in an electrical short between the anode catalyst support plate and the cathode catalyst support plate. Such a short will of course reduce the efficiency of the fuel cell and may cause localized overheating.
Finally, the seal configuration in a typical PEM cell generally employs use of an acrylic tape to bond the periphery of the water transport plates to the periphery of the catalyst support plates in order to create an effective seal between these components. Such acrylic tape is also used to bond the two halves of the water transport plate to each other. Such seal configurations exhibit marginal performance due to hydrophobicity of the materials that are used and due to poor adhesion with the PEM. Further, such seals are expensive to manufacture as a result of the number of components and process steps required in their fabrication and assembly.
One object of the present invention is to provide a singe peripheral sealant coating that adheres to the porous graphite water transport plates and the porous graphite catalyst support plates, and which also adheres to the exchange membrane itself.
An advantage of the present invention can be attributed to the fact that this single adhesive/sealant that is used to coat the exterior edges and to impregnate the exterior edges of these components provides a convenient way of achieving desired functions of both sealing and of adhesion, and also obviates the disadvantages created by use of the materials described above for achieving the sealing and adhesion functions.